Active VCF Design (Part 1)

So I have not fallen off the face of the earth. Been getting in the groove of the semester and haven’t set aside time to write till now. I have some updates to share!

My professor and I agreed to make this Arduino-Filter circuit an active filter. An active filter differs from a passive in that it allows gain. Which basically means the output signal can be bigger than the input signal. This is useful for me in that I don’t want the guitar signal to lose too much voltage going through the VCR stage, therefore I can include a “recovery” stage in my design. Cool!

This means my design needs to include some kind of operational amplifier. The LM471 is an op-amp I am familiar with and what I plan to use in prototyping and possibly final implementation.

So, let's dive into active filters! Below is the schematic of a typical Active Low-Pass Filter. (Courtesy of Wikipedia)



Just as my passive filter design over the summer, I need to integrate an FET into this circuit to make it voltage-sensitive. Now any Average-Joe-Engineering-Student knows from AC Circuit Analysis that the cut-off frequency of this circuit is controlled by the feedback impedance (C and R2) while the gain is set by the ration of R2 to R1. Therefore, the FET will need sit in parallel or series with R2 to vary the feedback resistance.

I failed to understand that last point in my first design. The circuit below isn’t a VCF at all!



In this design, the FET simply acted as a voltage-controlled voltage divider followed by a gain stage. Possibly useful but not what I needed.

I haven't drawn it up in PSPICE yet, but my next design corrected this and has the FET in the feedback section of the circuit. Much better! I'll include some schematics of this better design in my next post.

And we have arrived at my current position! Next step is to simulate, analyze, and implement the circuit  I have to see what happens.

As you saw in the August 27th post, the amplifier is complete! It is now just waiting for the VCF to be installed. If I have time, I would like to include a switch in the amp to select between analog EQ controls and my new VCF, that way I don’t lose the good old-school EQ controls forever with this modification.

I’ll be back with a report on that circuit soon. For now, bye friends!